1. Technical Field
The present invention relates to a photoelectric conversion element, a solid-state imaging element, an imaging apparatus, and a method for manufacturing a photoelectric conversion element.
2. Related Art
In general solid-state imaging elements having a photodiode within a semiconductor substrate, the pixel size reaches the limits of miniaturization, and an enhancement of performance such as sensitivity is becoming difficult. Then, there is proposed a stack type solid-state imaging element with high sensitivity in which a photoelectric conversion layer is provided above a semiconductor substrate, so as to enable one to achieve 100% of an aperture ratio (see JP-A-2008-263178).
The stack type solid-state imaging element described in JP-A-2008-263178 has a configuration in which plural pixel electrodes are arranged and formed above a semiconductor substrate, an organic material-containing light receiving layer (including at least a photoelectric conversion layer) is formed above the plural pixel electrodes, and a counter electrode is formed above this light receiving layer. In such a stack type solid-state imaging element, a bias voltage is impressed to the counter electrode, so as to add an electric field to the light receiving layer; a charge generated within the light receiving layer is transferred into the pixel electrodes; and a signal in response to the charge is read out by a read-out circuit connected to the pixel electrodes.
In the stack type solid-state imaging element, there may be the case where after forming the pixel electrodes, the light receiving layer and the counter electrode, for example, a protective film for blocking the outside air (e.g., water or oxygen), a color filter and other functional film, and so on are formed above the stack. In such case regarding a color filter, for example, the light receiving layer is coated with chemicals for the protective film, the color filter and other functional film, and also subjected to a heating step of heating generally at a temperature of about 200° C. for achieving curing.
Also, on the occasion of wire bonding for electrically connecting a substrate circuit and a package to each other and on the occasion of die bonding of chips to a package or solder reflow for connecting a package to an IC substrate, and the like, the heating step is performed. Furthermore, for achieving the wire bonding, it is necessary to provide a PAD opening in the chip circumferences and the like. On that occasion, resist pattern formation and etching are performed, and the substrate having the light receiving layer formed thereon goes through the heating step in each of the resist pattern formation step and the etching step.
In the light of the above, in the case where it is intended to fabricate a solid-state imaging element using an organic material-containing light receiving layer, when a processing method which is used for usual silicon devices is utilized, a high-temperature heating step is necessary, and the light receiving layer is required to endure such a heating step.
As a technique for enhancing the heat resistance of the light receiving layer, the use of a material having a small thermal change (for example, a material having a high glass transition temperature Tg) is generally applied. However, since the light receiving layer is required to have not only heat resistance but characteristics such as high photoelectric conversion efficiency and low dark current, it is necessary to select a material capable of satisfying these characteristics and heat resistance. In consequence, a width of selection of material of the light receiving layer is narrowed.
As described above, as the technique for enhancing the heat resistance of the light receiving layer, many techniques for improving the light receiving layer itself are proposed. However, any technique for enhancing the heat resistance while paying attention to constituent elements other than the light receiving layer has not been known yet.
Incidentally, even in not only the solid-state imaging element but other devices such as solar cells using a light receiving layer, so far as those prepared through the heating step after the formation of a light receiving layer are concerned, such a problem regarding the heat resistance is similarly generated.
JP-A-2005-085933, JP-A-2008-072435, and JP-A-2008-072589 describe a manufacturing method of a photoelectric conversion element in which ITO is film-formed on a glass substrate by means of sputtering and then subjected to patterning to form pixel electrodes, the substrate is heat dried at 250° C., and thereafter, a light receiving layer and a counter electrode are formed.
However, in this manufacturing method, only heating is performed at 250° C. for drying the ITO pixel electrodes, but an enhancement of the heat resistance is not aimed. Also, a specific configuration for enhancing the heat resistance is not described.
Also, JP-A-2009-071057 and JP-A-11-326038 describe that pixel electrodes are formed by a CVD method. However, these patent documents do not describe a specific configuration for enhancing the heat resistance.
Also, JP-A-2001-007367 describes a manufacturing method in which pixel electrodes are formed and then heated at 230° C. or higher. However, this patent document does not describe a specific configuration for enhancing the heat resistance.
In view of the foregoing circumstances, the invention has been made, and an object thereof is to provide a photoelectric conversion element including an organic material-containing light receiving layer, which is able to enhance heat resistance regardless of the material of the light receiving layer. Also, another object of the invention is to provide a solid-state imaging element equipped with this photoelectric conversion element, an imaging apparatus equipped with this solid-state imaging element and a method for manufacturing this photoelectric conversion element.